For a known electric device comprising insulated conductors operating at a high voltage, i.e. a voltage above 100 kV, such as a high-voltage transmission or distribution cable or a power transformer or reactor used in a network for transmission or distribution of electrical power it is known to either use an essentially solid insulation comprising a polymeric material or a porous material impregnated with a dielectric fluid, e.g. an insulation based on cellulose fibers and impregnated with an electric insulating oil. In this application, cellulose fibers mean pulp fibers which contain cellulose and to a varying extent lignin and hemi-cellulose.
Conventional cellulose-based electrical insulations consists of wound or spun layers of tape or of preformed bodies manufactured by dewatering and/or pressing a slurry comprising the cellulosic fibers, commonly known as pressboard. Both wound and preformed insulations are impregnated with an electrically insulating fluid, a dielectric fluid, usually an organic fluid such as an oil. This impregnation is normally carried out prior to, in connection to or after the insulation have been applied around the conductor or between conductors. The active part of the insulation is the cellulose fibers in the paper or the board. The oil protect the insulation against moisture pick-up and fills all pores and voids, whereby the dielectrically weak air is replaced by the oil. It is also known to use porous tapes and boards containing polymer-based man-made fibers in such insulations and also impregnate porous fiber-based insulations with similar dielectric fluids.
The impregnation of these porous fiber-based insulations is time consuming and in case of large volumes to be impregnated such as for long high-voltage direct current transmission cables these impregnation processes are carried out for days or weeks using a strictly controlled temperature cycle to ensure a complete and even impregnation of the fiber-based insulation.
To ensure a good impregnation result, a fluid exhibiting a low-viscosity is desired. But the fluid shall be viscous at normal operation conditions for the electrical device to avoid migration of the fluid in the porous insulation, and especially away from the porous insulation. Darcy's law is often used to describe the flow of a fluid through a porous media. ##EQU1##
In this law v is the so called Darcy velocity of the fluid, defined as the volume flow divided by the sample area, k is the permeability of the porous media, .DELTA.P is the pressure difference across the sample, .mu.is the dynamical viscosity of the fluid and L is the thickness of the sample. Thus the flow velocity of a fluid within a porous media will be essentially reciprocally proportional to the viscosity. A fluid exhibiting a low-viscosity or a highly temperature dependent viscosity at operating temperature will thus show a tendency to migrate under the influence of temperature fluctuations naturally occurring in an electric device during operation and also due to a temperature gradient building up across a conductor insulation in operation and might result in the formation of unfilled voids in the insulation. Both temperature fluctuations and temperature gradients in conductor insulation will be more expressed in high-voltage direct current devices such as HVDC cables than for most other electric insulations. Unfilled voids will in an insulation operating under an electrical high-voltage direct current field constitute a site where space charges tends to accumulate, thus risking the initiation of dielectric breakdown through discharges which will degrade the insulation and ultimately might lead to its breakdown. Unfilled voids in the insulation as a result of a poor impregnation will have the same effect as described in the foregoing. Thus a dielectric fluid is required that exhibit a low-viscosity under impregnation and is highly viscous under operation conditions.
Conventional dielectric fluid used for impregnating a porous conductor insulation comprised in an electric device, such as a cable, transformer or reactor used in an installation for high-voltage direct current transmission exhibit a viscosity that decreases essentially exponential as the temperature increases. Thus in the high temperature range for impregnation, the temperature has to be increased substantially to gain the required decrease in viscosity due to the low temperature dependence of the viscosity at these temperatures. In comparison the temperature dependence of the-viscosity; as at temperatures prevailing during operation conditions, is very high. Thus small variations in impregnation or operation conditions might have detrimental effect on the performance of the dielectric fluid and the conductor insulation. When using such dielectric fluids they can be chosen such that they are sufficiently viscous at normal operation temperatures to be essentially fully retained in the insulation also under the temperature fluctuations that occurs in the electric device during operation and also that this retention is unaffected of the temperature gradient that normally builds up over a conductor insulation for an electric device comprising conductors at high-voltage. This will mean that the impregnation will have to be carried out at a temperature substantially higher than the operation temperature the insulation is designed to operate at. The high impregnation temperature is needed to ensure that the insulation will be essentially fully impregnated. Such high impregnation temperatures are however disadvantageous as they risk effecting the insulation material, the surfaces properties of the conductor and promotes chemical reactions within and between any material present in the device which insulation is being impregnated. Also energy consumption during production and overall production costs will be negatively affected by a high impregnation temperature. Another aspect to consider is the thermal expansion and shrinkage of the porous insulation which implies that the cooling rate during cooling must be controlled and slow, adding further time to the already time consuming process. For a conventional insulating oil to exhibit a sufficient temperature dependent change in viscosity, a base oil in which a conventionally used polymer, e.g. polyisobuthene, is disolved in exhibits a highly temperature dependent viscosity. This can only be achieved for highly aromatic oils, such as the base oil of T2015 from Dussek Campbell. Such oils exhibits, however, poorer electric properties in comparison with more naphtenic oils, which are oil types suitable for us as insulation oil in an electric device according to the present invention. A more aromatic oil must additionally normally be treated with bleaching earth to exhibit acceptable electric properties. Such processing is costly and there is a risk that small sized clay-particles remains in the oil if not a careful filter- or separation-processing is carried out after this treatment. Alternatively an oil as disclosed in U.S. Pat. No. 3,668,128 can be chosen for its low viscosity at low temperatures. The oil described in U.S. Pat. No. 3,668,128 comprise additions of from 1 up to 50 percent by weight of an alkene polymer with a molecular weight in the range 100-900 derived from an alkene with 3, 4 or 5 carbon atoms, e.g. polybutene. This oil exhibit a low viscosity at low temperatures, good oxidation resistance and also good resistance to gassing, i.e. the evolution of hydrogen gas which might occur, especially when an oil of low aromatic content, as the oil suggested in U.S. Pat. No. 3,668,128, is exposed to electrical fields. The problem, how to retain this low viscosity oil in the cable insulation during the cyclic conditions as to temperature fluctuations or build up of a temperature gradient in the insulation which occurs in a cable or other conductor insulation that during operation is subjected to a high-voltage direct current field is not addressed in this publication. Thus a conductor insulation impregnated with an oil according to the disclosure in U.S. Pat. No. 3,668,128, although offering a major advance on the traditional electrical insulating oil for paper insulated cables, still suffers from the risk of voids being formed in the porous insulation due to migration caused by temperature fluctuations and or temperature gradient building up under operation.
In European Patent Publication EP-A1-0 23 1 402 a gel-forming compound is disclosed that exhibit a slow forming and thermally reversible gelling properties. The gel-forming compound is intended to be used as an encapsulant to ensure a good sealing and blocking of any interstices in the cable insulation such as unbonded interfaces or other internal spaces present between solid insulations, solid semi-conducting shields or layers and conductors in a cable insulated with solid polymeric insulation materials to avoid water from penetrating the insulation by intrusion and spreading along these internal interstices. This slow-forming thermally reversible gel-forming compound comprises an admixture of a polymer to a naphtenic or paraffinic oil and also embodiments using further admixtures of a comonomer and/or a block copolymer and is considered suitable as encapsulant due to its hydrofobic nature and the fact that it can be pumped into the interstices at a temperature below the maximum service temperature of the encapsulant itself. Similar gel-forming compounds for the same purpose, i.e. the use as encapsulant to block water from entering and spreading along interstices and internal surfaces in a cable comprising solid polymeric insulations, solid semi-conducting shields and metallic conductors are also known from the European Patent Publications, EP-A1-0 058 022 and EP-A1-0 586 158. In none of these publications no reference is, however, made to the specific demands put on an insulation for a conductor comprised in a high-voltage direct current apparatus, such as the need to essentially eliminate all unfilled voids or other inhomogenities. Nor is any reference made to the specific demands put on the liquid to fully fill essentially the whole porosity of a porous insulation for this application and be retained in this insulation as the temperature fluctuates and temperature gradients builds up during the operation of such an apparatus. Thus there is no reference of the possibility to use these gel-forming compounds as dielectric fluids in porous, fiber-based conductor insulations and especially not as to whether or not they would be suitable for use under the specific demands put on a dielectric fluid to be used for impregnating a fiber-based conductor insulation in a high-voltage direct current device.
It is an object of the present invention to provide an electric device which exhibit an insulation of its conductors that ensures stable dielectric properties and allows higher opertion temperatures without raising the impregnation temperature.
In particular it is the object of the present invention to provide an electric device as defined in the foregoing objective designed for operation under the specific conditions prevailing for high-voltage direct current devices.
It is therefore the object of the present invention to provide an electric device comprising an electric conductor with a conductor insulation in the form of a porous insulation impregnated with a dielectric fluid that;
exhibits a high viscosity and elasticity at temperatures within a first temperature range, comprising the temperature range in which the electric device is designed to operate such that the dielectric fluid will be essentially retained in the porous insulation at all temperatures in this range, PA1 exhibits a low viscosity at elevated temperatures within a second temperature range, comprising the temperature range deemed suitable and technically and economically favourable for impregnation, and PA1 that the viscosity within a third limited temperature range between said first and second temperature ranges changes from the high viscosity state exhibited within the first low temperature range to the low viscosity state exhibited within the second elevated temperature range. PA1 at temperatures within a first low temperature range is in a highly viscous and elastic, essentially gelled, state; PA1 at elevated temperatures within a second higher temperature range, is in low viscosity and essentially newtonian, easy flowing, state; and PA1 that over a third limited temperature range, the transition range, the viscosity of the dielectric fluid is changed between the low viscosity state and the highly viscous state. The fluid exhibit viscoelastic properties. The transition range comprises temperatures between the first and the second temperature ranges. PA1 that the block copolymer comprises at least one block in the block copolymer that exhibits a low solubility in the hydrocarbon-based fluid at temperatures within a first low temperature range, such that the block copolymer is only partly dissolved in the hydrocarbon-based fluid and a highly viscous and elastic gel is formed at temperatures within said first temperature range; PA1 that essentially all blocks in the block copolymer are soluble in the hydrocarbon-based fluid at elevated temperatures within a second higher temperature range, such that a fluid exhibiting low viscosity is formed at temperatures within said second temperature range; and PA1 that the solubility of one or more of the blocks in the block copolymer is changed substantially over a third limited temperature range, the transition range, which comprises temperatures between the first and the second temperature ranges, such that the viscosity of the dielectric fluid is changed between the low viscosity and the high viscosity states within over the transition range. PA1 that a part of the polymer when present in the hydrocarbon-based fluid exhibits a high tendency, at temperatures within a first low temperature range, to interact with the hydrocarbon-based fluid and to interact with the same part of other polymer molecules, thereby causing the formation of longer or more branched polymer molecules or cross-linking bridges in the fluid which thereby exhibit the flow properties of a highly viscous and elastic gel at temperatures within said first temperature range; PA1 that this tendency to form longer or more branched molecules or cross-linking bridges is substantially reduced at elevated temperatures within a second higher temperature range, such that a fluid exhibits low viscosity essentially newtonian at temperatures within said second temperature range; and PA1 that this tendency to form longer or more branched molecules or cross-linking bridges is substantially changed over a third limited temperature range, the transition range, which comprises temperatures between the first and the second temperature ranges, such that the viscosity of the dielectric fluid is changed between the low viscosity and the high viscosity states within over the transition range and exhibits viscoelastic properties.
This third temperature range shall be narrow to allow impregnation at a temperature closer to the operation temperature in comparision to a electric device impregnated with a conventional dielectric fluid.
It is further the object that the dielectric fluid shall exhibit a low temperature coefficient within both the first and second temperature ranges to ensure stable flow properties and flow behavior within these ranges, and that the change in viscosity within the limited third transition range is substantial, i.e. the change in viscosity is in the order of hundreds of Pas or more.